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Monthly values for the Arctic Oscillation Index, which tracks shifts in surface pressure between the mid-latitudes and the Arctic. During the positive phase, the jet stream stays farther north, and mid-latitude winters are milder than usual. Data from NOAA PSL.
Large map image for ARCTIC OSCILLATION

Winter surface pressure patterns across the Northern Hemisphere when the Arctic Oscillation (AO) was negative (2009-10) and when it was positive (1988-89). The negative AO favors a warm Arctic and cool conditions across the eastern U.S. Data from NOAA CPC.